(1) Field of the Invention
The present invention relates to an optical pickup apparatus which has a holographic optical element and a photodetector which are provided in common for light beams with different wavelengths used to access different types of optical storage media.
(2) Description of the Related Art
Currently, various types of rewritable optical disk drive are known, for example, a write-once optical disk drive which accesses a CD (compact disk recordable) and a rewritable optical disk drive which accesses a DVD (digital video disk). An optical pickup device of the write-once optical disk drive reads data from the CD, and can write data to the CD once only. An optical pickup device of the rewritable optical disk drive reads data from the DVD, and can write or erase data to the DVD many times.
Generally, a standard DVD has the recording surface under a transparent substrate which is about 0.6 mm thick, and a standard CD has the recording surface under a transparent substrate which is about 1.2 mm thick. In a DVD pickup device, a laser diode which emits a laser beam having a wavelength of 650 nm is used as the light source to access the DVD. In a CD pickup device, a laser diode which emits a laser beam having a wavelength of 785 nm is used as the light source to access the CD.
As disclosed in Japanese Utility Model Publication No.7-3461, an optical pickup apparatus for recording or reproducing of information of one of a first optical disk and a second optical disk in a shared manner is known. As described above, the first and second optical disks have the transparent substrates which are different in thickness.
In the optical pickup apparatus of the above publication, first and second laser sources selectively emit one of first and second laser beams, the first and second laser beams being different in wavelength, the wavelengths of the first and second laser beams being appropriate for accessing the first and second optical disks respectively. A reflection-beam separator which is configured with a prism of a certain type receives a reflection beam of a light spot from one of the first and second optical disks which is actually illuminated, and directs the reflection beam in one of predetermined directions depending on the wavelength of the reflection beam.
Further, in the optical pickup apparatus of the above publication, a first photodetector is provided to receive the reflection beam (having the wavelength of the first laser beam) from the reflection-beam separator, and to output a signal indicative of an intensity of the received reflection beam. A second photodetector which is provided separately from the first photodetector receives the reflection beam (having the wavelength of the second laser beam) from the reflection-beam separator, and outputs a signal indicative of an intensity of the received reflection beam.
In the optical pickup apparatus of the above publication, a focusing error signal and a tracking error signal can be generated based on the signal output by a corresponding one of the first and second photodetectors. Hence, the recording or reproducing of information of one of the first optical disk and the second optical disk can be achieved by the optical pickup apparatus of the above publication.
However, the optical pickup apparatus of the above publication must be configured with the first and second photodetectors which are provided independently of each other. The configuration of this apparatus is comparatively complicated, and it is necessary to provide a separate signal detection circuit for each of the first and second photodetectors. This makes the conventional optical pickup apparatus bulky and expensive, and it is difficult to achieve the manufacture of a small-size optical pickup apparatus with low cost.